The present invention relates generally to architecture of recording and reproducing information to and from optically readable storage media (referred to as the "disks" hereinafter), and more particularly to a method and apparatus for detecting position of alternated land and groove track of an information recording medium corresponding to a certain disk as formatted to accommodate advanced high-density information storage techniques.
Reproducing of information is performed by focussing a beam of laser light onto the data recorded layer of an optical disk and then detecting its reflection light modulated by a series of recording marks or "pits" as defined in the data layer. The recording marks or pits are recorded in a track as formed along a spiral groove track formed in the data layer. To increase the recording density, it is required that a disk-shaped optical storage medium be of high density in the thickness direction, circumferential direction, or radial direction thereof. One currently available approach to increasing the radial recording density is to decrease or narrow the track pitch of read/write tracks; unfortunately, this approach suffers from a problem in that such track pitch reduction has a limitation due to fabrication process of groove tracks.
Another approach to high density recording is disclosed in Japanese Patent Publication No. 4-4661, wherein an optical disk proposed therein is designed to render the width of a groove track per se substantially identical to the width of part that lies between adjacent groove tracks while setting the depth of groove tracks at a carefully selected value which minimizes cross-talk from neighboring tracks--the crosstalk is determinable by the wavelength of a laser beam irradiating the groove track depth--thereby permitting formation of recording marks between neighboring ones of the groove tracks in a similar way. With this approach, the use of recording marks provided between groove tracks may enable recording and reproduction of information, which in turn leads to the capability of improving the radial recording density of optical disks. A track provided in the groove tracks is called the "groove track," whereas another track as defined between groove tracks is known as the "land-portion track."
Continuous reproduction of information being recorded at the groove portion and land portion is as follows: in an optical disk with a continuous pattern of groove tracks being simply formed on the data layer from a point near the inner diameter toward the other point near the outer periphery thereof, certain time lag might take place when the operation mode is switched from continuous groove reproduction to continuous reproduction of land portions, or vice versa; this in turn makes it impossible, or at least very difficult, to continuously play back intended information of a land portion and that of its associated groove.
One typical approach to avoid the "continuous land/groove information reproduction incapability" problem is disclosed in Published Unexamined Japanese Patent Application ("PUJPA") No. 7-141701, wherein the groove tracks are formed in a way such that the tracks of land portions and grooves appear alternately for about every one circumference or "revolution" to thereby enable continuous reproduction of the prerecorded information at the land portions and grooves. An optical medium of this type is known in some cases as the "single spiral" disk among those skilled in the art to which the invention pertains.
In optical disks employing the single-spiral groove-track scheme, modulated position information is stored by formation of a train of pits on the track, and the read/write position is identifiable by reproducing such pit information for detection of a position alternated land and groove track at which a change occurs between the land portion and groove. However, this prior art approach does not come without accompanying a problem which follows: it remains difficult to form the pit information with respect to every track with high density and is also difficult to attain reproduction and demodulation with intended precision due to unwanted affection of crosstalk from a pit array formed in its neighboring tracks.
A still another prior art approach is disclosed in PUJPA No. 6-176404, which permits reproduction of information as to the position alternated land and groove track at both the land portion and the groove by forming pits in a way such that it is positionally shifted or "offset"--inwardly or outwardly on the disk layer--by a predefined distance equivalent to a quarter (1/4) of the center-to-center pitch of one land portion and its neighboring land portion.
A further prior art approach is disclosed in PUJPA No. 8-221821, which proposes pre-formatting the pits in the circumferential or tangential direction of tracks at substantially equal intervals and forming the pits so that these are alternately offset, inwardly and outwardly, from the center of the land portion or groove by a distance equivalent to 1/4 of the center-to-center pitch of adjacent land portions. The "alternate offset pit formation" approach enables an optical disk of this type to form specific information capable of relatively identifying the position alternated land and groove track at some physical identification (PID) sections other than a PID section for use in forming a pit(s) corresponding to the position alternated land and groove track.
For reproduction of the recording marks formed on the optical disk with such land and groove portions, it should be required that a spot of laser beam falling onto the data layer thereof scan along approximately the center position of a land portion or groove relative to the radius direction of the "land/groove alternate offset" optical disk, while at the same time requiring retainment of exact focussing on the data layer. In this case, the magnitude of a deviation (tracking deviation amount) which is substantially proportional to the distance of a laser beam spot from the center of either the land portion or the groove may be acquired by a tracking error signal (high-frequency push-pull or "HPP" signal) obtainable by detection of diffraction light components of the laser beam from groove tracks, for controlling (tracking control) the light collection or "focus" position of the laser beam by use of such deviation amount, wherein the deviation amount is inverted in polarity between the land portion and groove.
Accordingly, the tracking control used must be designed so that the deviation amount of the tracking error signal is inverted in polarity between the land portion and groove.
FIG. 10 is a schematic diagram showing part of the data layer of a disk-like optical medium layer near the PID section. Numeral "304" designates an HPP signal waveform based on the tracking deviation amount as generated when a beam spot virtually moves transversely with respect to the radial direction 1003 of the disk medium. As apparent from this diagram, the HPP signal is different in polarity between the land portion and groove. More specifically, when the HPP signal is negative in polarity for the land portion, it is to be judged that the beam spot is positionally deviated in the right direction of the plane of drawing of FIG. 10 relative to a groove track center 1001 thus performing a tracking control by causing the beam spot to move in the left direction of the plane of drawing; for the land portion, when the HPP signal is negative, then this tells that the beam spot is deviated in the left direction of the plane of drawing relative to a land track center 1002 thus effectuating the tracking by causing the spot to move rightward on the drawing plane.
Consequently, for continuous or "seamless" reproduction of information while the spot moves from the land portion to the groove or vice versa, it should be needed to control the focus position of laser beam in responding to occurrence of polarity inversion of the deviation amount at a specific position whereat a beam-focussed surface portion changes between the land portion and groove--namely, at the position alternated land and groove track. To do this, certain areas are defined with substantially equal intervals on the tracks of disk in the tangential direction thereof then forming pits indicative of the position alternated land and groove track in such areas, as taught from PUJPA Nos. 6-176404 and 8-221821. These pits are formed in a way such that reproducing the pit information renders recognizable the specific position (position alternated land and groove track) whereat the land and groove portions change therebetween while simultaneously permitting a switching of the status of tracking control, focus control and the like based on resultant information read therefrom.
One problem encountered with the approach to reproduction--by forming pits that lie midway between the land and groove portions and then reading the pit information by use of a radio frequency (RF) signal based on a change in total quantity of reflection light for effectuation of intended tracking control and focus control--is incapability of attaining an increased signal amplitude due to "offset" formation of the pits from the track center. This in turn raises a problem of position alternated land and groove track detection difficulty while rendering difficult any successful execution of the tracking control and focus control, resulting in noticeable difficulty of reproduction of recorded information.
Another problem faced with the prior art is that with the position alternated land and groove track detection by bit information reproduction, the position alternated land and groove track will no longer be recognizable in cases where the reproduction of pits is difficult due to pit formation defects, disk shape irregularity, or several kinds of unwanted control variations.
A further problem of the prior art is that the position alternated land and groove track becomes unpredictable upon erroneous detection of a plurality of PID sections because of the fact that the position alternated land and groove track is defined by its relative positional relationship with other PID sections. Also where the number of rotation or the linear velocity of disk medium is rapidly varied due to access operations, the phase-locked loop (PLL) control of a reproduced signal becomes unattainable making it impossible to demodulate and reproduce the pit information.